1. Field of the Invention
The current invention relates to clothing for paper machines and, in particular to, nonwoven clothing for the support and transport of a fibrous web in paper machines.
2. Description of the Related Art
Paper machines serve to produce fibrous webs, for example paper webs of various types, cartons, cardboards and similar nonwoven materials. The term “paper” is used in this documentation as being representative for these types of fibrous webs.
The production of a fibrous web begins in the forming section of a paper machine by bringing a fibrous stock suspension onto clothing or, respectively, by introducing a fibrous stock suspension into the gap which is formed between two layers of clothing. Clothing is normally in the embodiment of continuous belts which rotate always within a certain section of the paper machine, turned over rollers. The paper-side surface of the clothing carries the fibrous stock suspension or, respectively, the fibrous or nonwoven web created by the suspension. In the following description the surface of the clothing running over the rollers is referred to as the running side. The clothing has passages for dewatering through which the water can be sucked from the paper side toward the running side.
The clothing in the embodiment of forming fabrics which are currently used in the forming section of paper machines consists of woven material. Woven clothing, due to its uniform weave structure, frequently leads to undesirable marking in the paper web and due to its low flexural rigidity tends to wrinkle during rotation in the paper machine. Moreover, the woven seam of woven clothing in the embodiment of a continuous belt is extremely complex and cost-intensive. There is therefore a requirement for alternative clothing.
As an alternative to woven clothing, clothing was suggested that is produced from film-like nonwoven material. A clothing is cited, for example, in patent documents CA 1 230 511 and U.S. Pat. No. 4,541,895 which is formed from a laminate consisting of several layers of nonwoven water-impermeable materials into which openings for dewatering were introduced. Interconnection of the individual laminate layers occurs flat through, for example ultrasonic welding, high frequency welding or thermal welding. The dewatering apertures are created in the laminate preferably by means of laser drilling.
In patent application publication US 2010/0230064 a film-like clothing produced from a spirally wound polymer belt for use in paper machines is suggested. The width of the polymer belt is substantially less than the width of the clothing produced thereof, whereby the longitudinal direction of the polymer belt—with the exception of the tilt caused by the winding height—is consistent with the direction of travel of the clothing. The lateral edges of adjacent winding sequences of the polymer belt respectively located opposite each other are welded together to create a closed running surface. The clothing moreover has apertures through which air and/or water can move from one surface of the clothing to the other.
To avoid undesirable markings on papers the clothing must have homogeneous water permeability in those regions where they transport the fibrous web. With film-like clothing, aperture patterns are required wherein apertures having diameters in the range of approximately 50 to 250 micrometers (μm) are distributed at approximately even distances. An appropriate perforation however causes a weakness in the mechanical strength of the film-like clothing which can lead to tearing, in particular at the heavily stressed lateral edges.
In a known method the edges of woven forming fabrics are welded together through ultrasonic welding or thermal energy, possibly also strengthened with additives, for example polyurethane, so that the edges do not tear during rotation in the paper machine. Due to the lack of a woven structure whose threads can be joined with each other through welding and whose pores can be filled with additives, these methods of edge strengthening cannot be transferred to film-like clothing.
What is needed in the art is an edge configuration for film-like clothing for use in a paper machine which offers high reliability in the prevention of tearing of the clothing edge, while being utilized as intended.